ENH: add json schema

[Alvaro-Kothe]

Closes #1585

Too check the schema locally

1. Apply this patch:

diff --git a/asv.conf.json b/asv.conf.json
index ab1e6d6..f5f760e 100644
--- a/asv.conf.json
+++ b/asv.conf.json
@@ -1,4 +1,5 @@
 {
+    "$schema": "./asv.conf.schema.json",
     // The version of the config file format.  Do not change, unless
     // you know what you are doing.
     "version": 1,

2. Setup a json lsp, I used the one from microsoft

---

Gemini generated the initial draft, then I reviewed and modify according to the documentation in https://asv.readthedocs.io/en/latest/asv.conf.json.html